Secreted Proteins
Many human proteins serve as pharmaceutically active compounds. Several classes of human proteins that serve as such active compounds include hormones, cytokines, cell growth factors, and cell differentiation factors. Most proteins that can be used as a pharmaceutically active compound fall within the family of secreted proteins. It is, therefore, important in developing new pharmaceutical compounds to identify secreted proteins that can be tested for activity in a variety of animal models. The present invention advances the state of the art by providing many novel human secreted proteins.
Secreted proteins are generally produced within cells at rough endoplasmic reticulum, are then exported to the golgi complex, and then move to secretory vesicles or granules, where they are secreted to the exterior of the cell via exocytosis.
Secreted proteins are particularly useful as diagnostic markers. Many secreted proteins are found, and can easily be measured, in serum. For example, a ‘signal sequence trap’ technique can often be utilized because many secreted proteins, such as certain secretory breast cancer proteins, contain a molecular signal sequence for cellular export. Additionally, antibodies against particular secreted serum proteins can serve as potential diagnostic agents, such as for diagnosing cancer.
Secreted proteins play a critical role in a wide array of important biological processes in humans and have numerous utilities; several illustrative examples are discussed herein. For example, fibroblast secreted proteins participate in extracellular matrix formation. Extracellular matrix affects growth factor action, cell adhesion, and cell growth. Structural and quantitative characteristics of fibroblast secreted proteins are modified during the course of cellular aging and such aging related modifications may lead to increased inhibition of cell adhesion, inhibited cell stimulation by growth factors, and inhibited cell proliferative ability (Eleftheriou et al., Mutat Res 1991 March–November;256(2–6):127–38).
The secreted form of amyloid beta/A4 protein precursor (APP) functions as a growth and/or differentiation factor. The secreted form of APP can stimulate neurite extension of cultured neuroblastoma cells, presumably through binding to a cell surface receptor and thereby triggering intracellular transduction mechanisms. (Roch et al., Ann N Y Acad Sci 1993 Sep. 24;695:149–57). Secreted APPs modulate neuronal excitability, counteract effects of glutamate on growth cone behaviors, and increase synaptic complexity. The prominent effects of secreted APPs on synaptogenesis and neuronal survival suggest that secreted APPs play a major role in the process of natural cell death and, furthermore, may play a role in the development of a wide variety of neurological disorders, such as stroke, epilepsy, and Alzheimer's disease (Mattson et al., Perspect Dev Neurobiol 1998; 5(4):337–52).
Breast cancer cells secrete a 52K estrogen-regulated protein (see Rochefort et al., Ann N Y Acad Sci 1986;464:190–201). This secreted protein is therefore useful in breast cancer diagnosis.
Two secreted proteins released by platelets, platelet factor 4 (PF4) and beta-thromboglobulin (betaTG), are accurate indicators of platelet involvement in hemostasis and thrombosis and assays that measure these secreted proteins are useful for studying the pathogenesis and course of thromboembolic disorders (Kaplan, Adv Exp Med Biol 1978;102:105–19).
Vascular endothelial growth factor (VEGF) is another example of a naturally secreted protein. VEGF binds to cell-surface heparan sulfates, is generated by hypoxic endothelial cells, reduces apoptosis, and binds to high-affinity receptors that are up-regulated by hypoxia (Asahara et al., Semin Interv Cardiol 1996 September;1(3):225–32).
Many critical components of the immune system are secreted proteins, such as antibodies, and many important functions of the immune system are dependent upon the action of secreted proteins. For example, Saxon et al., Biochem Soc Trans 1997 May;25(2):383–7, discusses secreted IgE proteins.
For a further review of secreted proteins, see Nilsen-Hamilton et al., Cell Biol Int Rep 1982 September;6(9):815–36.
Hemopexin
The novel human protein, and encoding gene, provided by the present invention is related to hemopexin proteins. Hemopexins are globulins (beta-glycoproteins) that are synthesized in the liver and represent 1.4% if total serum protein. Each hemopexin molecule binds a single heme molecule with high affinity and transports the heme to hepatocytes for transfer of iron. Hemopexin levels are low in individuals with hemolysis.
Due to their importance in hematological physiology, particularly in regulating transportation of heme and iron, novel human hemopexin-related proteins/genes, such as provided by the present invention, are valuable as potential targets and/or reagents for the development of therapeutics to treat hematological diseases/disorders such as hemolysis and anemia, as well as other diseases/disorders. Furthermore, SNPs in hemopexin-related genes may serve as valuable markers for the diagnosis, prognosis, prevention, and/or treatment of such diseases/disorders. Using the information provided by the present invention, reagents such as probes/primers for detecting the SNPs or the expression of the protein/gene provided herein may be readily developed and, if desired, incorporated into kit formats such as nucleic acid arrays, primer extension reactions coupled with mass spec detection (for SNP detection), or TAQMAN PCR assays (Applied Biosystems, Foster City, Calif.).
For a further review of hemopexin, see Law et al., Genomics 3 (1), 48–52 (1988); Altruda et al., J. Mol. Evol. 27 (2), 102–108 (1988); Altruda et al., Nucleic Acids Res 1985 June 11;13(11):3841–59; Cai et al., Am. J. Hum. Genet. 39: A191 only, 1986; Kamboh et al., Am. J. Hum. Genet. 41: 645–653, 1987; Lush, “The Biochemical Genetics of Vertebrates Except Man.” Philadelphia: W. B. Saunders (pub.) 1966; Naylor et al., Somat. Cell Molec. Genet. 13: 355–358, 1987; Roychoudhury et al., “Human Polymorphic Genes: World Distribution.” New York: Oxford Univ. Press (pub.) 1988; Stewart et al., Ann. Hum. Genet.35: 19–24, 1971; and Takahashi et al., Proc. Nat. Acad. Sci. 82: 73–77, 1985.
Secreted proteins, particularly members of the hemopexin protein subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of secreted proteins. The present invention advances the state of the art by providing previously unidentified human secreted proteins that have homology to members of the hemopexin protein subfamily.